The invention relates to an apparatus for controlling the composition of the fuel-air mixture fed to an internal combustion engine. More particularly, the mixture control device includes a bypass line for providing bypass air to the induction tube of the engine downstream of the mixture generator, for example a carburetor. Within the bypass line there is disposed a throttle member, the movable part of which is actuated pneumatically in accordance with the vacuum provided by a fixed venturi within the main induction tube. The vacuum normally acting on the moving element of the throttle can be replaced by some other pressure via a valve assembly that is itself controlled by a transducer which senses an operational variable of the engine.
In a known apparatus of this general type, the bypass line contains a throttle member, the movable part of which is a control diaphragm which is loaded on both sides by compression springs. This control diaphragm defines two pressure chambers, one of which is connected to a storage volume and to the narrowest portion of a venturi within the induction tube of the engine. By actuating a magnetic valve in a connecting line leading to the atmospheric air, the induction tube vacuum derived there may be changed. The other pressure chamber of the throttle member is connected with the induction tube downstream of the main throttle plate which defines the amount of mixture fed to the engine. By suitable cyclic actuation of the magnetic valve within the connecting line to the ambient air, and in dependence on a signal from an oxygen sensor located in the exhaust system of the engine, and/or by suitable construction of the throttle member itself, it is possible to control the supply of secondary air to be dependent on the air throughput rate through the main induction tube. Furthermore, a final correction based on the signal from the oxygen sensor is possible by changing the venturi vacuum. In this known apparatus, it is further possible to obtain an additional reduction of the bypass air quantity in dependence on the pressure in the induction tube downstream of the throttle plate. However, this known apparatus is capable of only relatively coarse control of the secondary air stream. In particular, it is unable to guarantee that the flow rates within the induction tube and in the bypass line are strictly proportional to one another. This is so because, while the throttle member is displaced as a function of the throughput, the pressure drop across the throttle does not correspond to the conditions within the induction tube. On the other hand, it is a first and principal requirement for effective control in all domains of operation of the engine to provide a proportional preliminary control of the flow rate of the supplied secondary air stream. The known apparatus does not guarantee a multiplicative control influence. In the known apparatus, furthermore, when the throttle plate is closed, the sharply increasing vacuum in the induction tube downstream of the throttle plate either very sharply diminishes or completely interrupts the supply of secondary air.